CMP represents a major portion of the production cost for semiconductor wafers. These CMP costs include polishing pads, polishing slurry, pad conditioning disks and a variety of CMP parts that become worn during the planarizing and polishing operations. The total cost for the polishing pad, the downtime to replace the pad and the cost of the test wafers to recalibrate the pad is approximately $7 for a single wafer polishing run. In many complex integrated circuit devices, up to five CMP runs are required for each finished wafer which further increase the total manufacturing costs for such wafers.
The greatest amount of wear on the polishing pads is the result of polishing pad conditioning that is necessary to place the pad into a suitable condition for these wafer planarization and polishing operations. A typical polishing pad comprises a closed-cell polyurethane foam approximately 1/16 inch thick. During pad conditioning, the pads are subjected to mechanical abrasion to physically cut through the cellular layers of the surface of the pad. The exposed surface of the pad contains open cells which trap an abrasive slurry consisting of the spent polishing slurry and material removed from the wafer. In each subsequent pad conditioning step, the ideal conditioning head removes only the outer layer of cells containing the embedded materials without removing any of the layers below the outer layer. Such an ideal conditioning head would achieve a 100% removal rate with the lowest possible removal of layers on the polishing pad, i.e., lowest possible pad wear rate. It is apparent that a 100% removal rate can be achieved if there were no concern for its adverse affect of wear on the pad. However, such over-texturing of the pad results in a shortening of the pad life. On the other hand, under-texturing results in insufficient material removal rate during the CMP step and lack of wafer uniformity. Using the prior art conditioning heads that achieve satisfactory removal rates, as low as 200 to 300 and as high as several thousand wafer polishing runs, depending on the specific run conditions, can be made before the pad becomes ineffective and must be replaced. This occurs after the pad is reduced approximately to half of its original thickness.
There is a great need for a conditioning head that achieves as close to the ideal balance between high wafer removal rates and low pad wear rate so that the polishing pad effective life can be significantly increased without sacrificing the quality of the conditioning.
The prior art conditioning heads typically comprise a stainless steel plate, a non-uniform distribution of diamond grit over the surface of the plate and a wet chemical plated over-coat of nickel to cover the plate and the grit. The use of such prior art conditioning heads is limited to the conditioning of polishing pads which have been used during oxide CMP wafer processing, i.e. when the exposed outer layer is an oxide-containing material as opposed to metal. In processing a semiconductor wafer, there are about the same number of oxide and metal CMP processing steps. However, the prior art heads are ineffective for conditioning metal processing operations. This is the case because the slurry used to remove metal from the wafer reacts with the nickel and degrades and otherwise dissolves the nickel outer layer of the conditioning head and causes a major loss of the diamond grit from the plate, potentially scratching the wafers.
There is a great need for a head that is effective in conditioning both oxide-containing and metal-containing wafer surfaces. There is also a great need for a conditioning head in which the diamond grit is more firmly attached to the underlying substrate. There is also a need for a conditioning head that provides a greater degree of uniformity of wafer material being removed from a given wafer during the CMP operation. Finally, there is a need for a conditioning head that extends the life of the polishing pads.